The present invention relates to a receiver and, more particularly, to a multiple frequency receiver having switching apparatus for selecting any one of a plurality of tuning elements to provide reception on one of a plurality of different channels of radiowaves.
In the receiver of the above described type, the channel elements may be selected by a manual selector switch or by some automatic control system. In some cases it is desired to accord a priority to one channel and to receive such priority channel at all times during which a signal is transmitted thereon. In the absence of a signal on the priority channel the receiver may operate on another channel or may operate on a plurality of other channels at different times.
For example, in a citizen band receiver, a plurality of channels are available for, communication, in which one channel is designated for emergency uses. Conventionally, there have been proposed various types of receivers which automatically sample the priority channel intermittently during the reception of the normal channels. One type of a conventional type receiver is disclosed in U.S. Pat. No. 3,614,621 to Ronald H. Chapman patented on Oct. 19, 1971, and U.S. Pat. No. 3,497,813 to Thomas Gallagher patented on Oct. 24, 1970, in which there is provided means for detecting the presence of a received signal by the detection of noise. When the receiver is particularly designed for receiving a frequency modulation (FM) signal, the level of the noise becomes considerably low during the receiver receiving an FM signal, and the level of the becomes considerably high during the receiver not receiving the FM signal. However, such difference in the noise level do not appear in the case where the receiver is particularly designed for receiving amplitude modulation (AM) signal.